Question

I just read the code for std::for_each:

template<class InputIterator, class Function>
Function for_each(InputIterator first, InputIterator last, Function f)
{
  for ( ; first!=last; ++first ) f(*first);
  return f;
}

and could not see any good reasons for this template function to return the input function. Does anyone have any examples on where this would be useful?

Answer 1

It’s to allow you to accrue state in your function and then return it to your calling code. For instance, your function (as a functor class) could have a member int for counting the number of times it had been called.

Here is a page with some examples : https://web.archive.org/web/20171127171924/http://xenon.arcticus.com/c-morsels-std-for-each-functors-member-variables